Day 6 takes you down the Rogue and up Graves and Wolf Creek, through the heart of the Rogue River mining district.
Southwest Oregon was once home to hundreds of gold mines, which operated from the later 1800s to the 1940s. F.W. Libbey, an Oregon State geologist, wrote in 1963, “Gold mining was originally the mainstay of the economy of southern Oregon. It started settlements, built roads and schools, promoted local government, and established law and order. It is now at best only a token of its past.” Despite that fact that a lot of gold remains in the ground, mining today is still a shadow of what it was a hundred years ago. During World War II, gold mines were closed by government decree, because the labor and equipment was needed to mine materials critical to the war effort, which gold was not.
There are two different types of gold mine in the area—lode and placer. In lode mines, gold occurs in narrow veins in the rock, typically mixed with quartz and sulfides, which are compounds of sulfur with metals like copper, lead, iron and zinc. Lode veins form when bodies of magma are injected into the crust of the earth and slowly cool. As the magma solidifies, chemical elements like sulfur, water, and metals are concentrated in fluids that percolate towards the surface through fractures in the rock. As the fluids cool, mineral crystals precipitate, filling the fractures with veins of ore. Mining lode gold is a difficult and dangerous process which involved excavating tunnels with dynamite to follow the veins.
Placer deposits on the other hand are much easier to mine. Placer gold forms when the rocks that encase ore veins are weathered and eroded away over millions of years. Gold is essentially indestructible at the surface of the earth, so that long after the mountain that held the vein has crumbled into sand and been washed to the sea, the gold contained in the veins is still fresh, and gets trapped in the sediment at the bottom of streams and rivers. Gold is one of the densest elements known, twice as dense as lead, so even very fine particles are hard for a river to move, and they accumulate in river gravel. There is still placer gold in most of the rivers of southern Oregon, including the Rogue, but most of the richest placer mines were in river terraces, like the ones you rode across on Day 5.
Lode gold mines are common in the hills surrounding Galice and Indian Mary Park, and you will see an obvious example at mile eight, just past the Almeda Park. You will pass a big road cut on the right made up of crumbly yellow-orange and red-stained rock. This is a zone of mineralized rock that hosts the ore veins that the Almeda mine followed, with thousands of feet of underground tunnels and shafts that have long since been closed or collapsed. The mine produced gold, silver, copper, and lead for many years in the 1930s and 1940s. As you ride down the Rogue and then turn up Graves Creek, you will be passing numerous old placer
Old placer mines along Graves Creek show as ragged pits on the smooth surfaces of old river terraces in this perspective view of lidar imagery.
mines on the ancient river terraces that now lie high above the modern streams. Mines with names like the Lucky Shot, Golden Light, and Vindicator recall a time when mining was the mainstay of the local economy. Using recent lidar topographic scanning data, we can see the scars left by large-scale placer mining in the days before reclamation of mined lands was required. These older mines were typically worked using hydraulic mining. Hydraulic mining relied on a network of ditches to bring water to the mine from surrounding creeks, and the water was then sprayed at high pressure through a giant iron nozzle, called a monitor. The resulting stream tore away the loose gravel holding the gold and sluiced the material into boxes designed to trap the gold and pass the remaining sediment out. This was a very cheap and effective form of mining, but left damaging amounts of sediment in streams and rivers, and it is no longer allowed.
After you leave Wolf Creek and begin the climb along I-5, you will have plenty of time to admire a huge road cut along the interstate. The road cut is composed of a rock geologists call greenstone, largely because it is green. Greenstone is a catch-all term for a wide range of metamorphosed volcanic rocks, which typically are green because they contain two iron-rich metamorphic minerals, chlorite and epidote. Iron is one of the most common sources of strong colors in rock, producing yellow-orange and red when it is in the oxidized form, and green when it is reduced. You will see more road cuts of greenstone along the road to the lunch stop and the return to Glendale.
Geology Rocks Day 7: Fire and Faults
The ride down Cow Creek will take you through the heart of the 2013 Douglas Complex fire. Areas shown in red were severely burned. Orange means moderately burned, and green lightly burned. You will cross the Canyonville Fault and pass beneath Nickel Mountain, site of the only nickel mine in the US.
The final day of riding takes you down the valley of the beautiful Cow Creek. For much of the day, the geology will be the familiar rocks of the exotic terranes, mélange, greenstone, and serpentinite.
From 6 to 10 miles into the ride, you will enter a long stretch that was severely burned in 2013 by the Douglas Complex Fire. This fire, which was started by lightning, covered over 44,000 acres and was controlled thorough the efforts of over 3,000 firefighters. The fire may have cleared away enough vegetation that you will be able to see some old placer mines on both sides of the road at about mile 8. These mines, on high river terraces, produced gold and in some cases platinum and chromite. Platinum and chromite are minerals that are found in ultramafic rocks, so it is no surprise to find them in placer deposits where the bedrock includes serpentinite. You will see the Douglas Complex fire again from miles 12 to 20.
At mile 25, you will cross a major fault, called the Canyonville Fault, which extends for 50 miles east to west through the area. When you cross the fault, you will once again see the thick sedimentary turbidite layers that you saw in the Tyee country on Day 2. But the fault is actually very complex, so you will pass through alternating slivers of exotic terranes, and thick beds of turbidite sandstone and conglomerate. You will see a great outcrop of tilted turbidite layers at about mile 32, just before you come out into the wide valley at Riddle.
In this perspective view derived from lidar imagery, tilted sandstone and conglomerate layers are clearly visible from the road near mile 32.
Shortly before you turn off Cow Creek Road onto Glenbrook Loop Road, you will see what appears to be a large industrial facility to the left, with big piles of crushed rock, and you may notice a large bare patch at the top of the mountain behind the stockpiles. This is the remnant of the Hanna Nickel mine, which is the only significant nickel mine to have operated in the US. Despite having relatively low grade ore, the mine was opened with a US government subsidy because of concern over dependence on foreign sources of nickel, a strategic metal. When subsidies were halted in the 1980s, the smelter continued to produce nickel form higher grade imported ore, but it was ultimately closed and sold for scrap.
Ian Madin is the Chief Scientist for the Oregon Department of Geology and Mineral Industries and in his spare time provides geology blogs for the Cycle Oregon. He will be providing commentary during the evening program and will be a available to answer questions during the ride.